Telephone-exchange system



R. S. BAILEY.

TELEPHONE EXCHANGE SYSTEM.

APPLICATION FILED MAR. 22, 1921.

1,434,5 1 8 Patented. Nov. 7, 1922;.

2 SHEETSSHEET I.

MARGINAL R s. BAILEY. TELEPHONE EXCHANGESYSTEM.

APPLICATION FILED MAR. 22,192].

1,434,518, Patented Nov. 7, 1922;.

2 SHEETS-SHEE'I 2. I

Patented Nov. 7, ll22= RAND S. BAILEY, OF BEONTCLAIB, NEW JERSEY, rLSSIGNOI-t TO ABEIERIGAH EFELEEHONE AND TELEGRAPH GOSMEPA'N'EE, A CGRPORATEQN 0F NEW YORK.

TELEPHONE-EXCHANGE SYSTEM.

Application filed March .22, 1921.

To all on 71 cm it may concern.

Be it known that l, RAN!) S. BEJLIWY, residing at'liflontclair, in the county oi Essex and State of New Jersey, have invented certain Improvements in Telephone-Exchange ii gtems, of which the following is a specitication.

This invention relates to telephone exchange systems and more particularly to circuit arrangements for use at a private branch exchange when such an exchange is used in connection with a main central ollice, which central ofliee may be either of the manual or machine switching type.

An object oi the invention is to provide an improved cord circuit for use at a private branch exchange to interconnect subscribers of the private branch exchange and to corn nect subscribers of the private branch exchange with trunk lines leading to the main central otlice. wherein through supervision is provided on outgoing calls over the trunk line and non-through supervision is provided on incoming calls over the trunk line.

A. feature of the invention consists in automatically converting the cord circuit from one class of supervision to the other ly ineaus of apparatus in the cord circuit reoo erating with apparatus in the connected trunl-I circuit For a more complete the invention. reference may be had to the accompanying drawings in which Figure 1 illustrates one embodiment of the invention applied to a cord circuit C arranged for single, supervision when it isvernployed for interconnecting lines of the private branch .1 :change, such as L and L and in which l 2 lllllSllIutES another embodiment of the invention applied to a cord circuit C arranged, tor double supervision when it is employed tor interconnecting extension lines ol the private branch exchange, such as E and ll), and arranged to split the cord circuit and to provide a recall signal when a second call is received over a trunk line T lie'lore the operator disconnects.

liPlCIllil II to Fi l oi the drawings and assuming tut a subscriber at the main central ol'lice desires to be connected to a sub- :--criber one oi the extensions as, for example. the exte sion ll of the private branch exchange, ringing current will be tronsmittwl over the trunk line T from the main central ollice, tlticreby energizing the understanding of Serial No. 45 1594.

relay 1i and completing an obvious circuit private branch exchange, upon observing the trunk signal 12, may insert a plug 15 into the jack 10, whereupon a circuit is completed follows: ground, winding of relay 18, upper winding of relay 1?, inner lefthand normal contacts of relay 16, sleeves of plug 15 and a jack 10 and thence in parallel, one path extending through resistance to grounded battery and the other path cxtene'ng through the right-hand contacts and winding of relay 1% to grounded battery. Relay ll, is slow-to-operate and slow-to-release and has its windings differentially woun l, The flow of current through both windi or the differentially wound relay let causes relay 1% to release. Due to the momentary lowered resistance of the sleeve circuit by reason of the path through toe ri it-hand winding of the ditl'erential relay 1., when the plu 15 is inserted into jack l0, sufficient current passes through the upper winding of relay 17, which is (it the marginal type, to energize said relay 17. As

soon as relay 14 releases, the resistance or":

plied to the tip and ring conductors of the cord circuit C over the upper and lower back contacts of relay l9 and through the windinns oi the low impedance coils and 21.

il heu the relay 19 is energized however, t ie battery supply connections are opened and the tip and ring conductors leading to the to are connected together through the coils 920 and 21 by a connection extending through the upper alternate contacts of relay 19 and outer left-hand normal contacts of relay l6. A low impedance bridge is thus estal'z-lished across the cord circuit -for tripping the machine ringii'ig apparatus at the main central office, in the manner well known in the art.

The operator may now actuate the key LK in order to connect her set with the cord ci cuit, )Vhen the key LK is in normal condition, the tip conductors of the front and back cords are connected by a path extending from the tip contact of the plug 15, over the upper normal contacts of keys FK, LE and BK to the tip contact of the plug 50. In a similar manner a path extends from the ring contact of the plug 15, lower normal contacts of key FK, lowermost normal contacts of key LK, winding of supervisory relay 34 and the serially connected non-inductive resistances 30 and 31 in parallel, lower normal contacts of key BK tothe ring contact of plug 50. )Vhen the key LK is actuated, however, the circuit above described is changed so that the tip conductors of the front and back cords are connected over a path extending through the upper innermost contacts of key LK, upper normal contacts of relay 58, and thence through contacts 90 of key LK. Ring connections on the other hand extend through the lowermost alternate contacts of key LK, lower normal contacts of relay 58, and uppermost contacts of key LK. The operators talking and listening set comprising a trans mitter M and a receiver B respectively, and the associated induction coil 98 and repeating coil 99 are bridged across the circuit so that the operator is enabled to converse with the calling subscriber or operator at the distant central office to determine what extension is desired.

The operator, upon obtaining this information and ascertaining that a connection to the extension L is desired, restores the listening key LK to normal and inserts the plug 50 into the jack 51, thereby closing a circuit from ground over the sleeve contacts of the jack 51 and plug 50, arma ture and back contact of relay 34, and

through the supervisory lamp 40 to grounded battery. The lamp 40 is now caused to glow. The operator may then actuate the ringing key BK to transmit ringing current to the desired subscriber.

Upon the response of the called subscriber at the station associated with the extension L current from the main central oflice flows through the station of the extension L causing the cord circuit supervisory relay 84 to operate and the low impedance bridge to be removed from the cord circuit. This circuit may be traced as follows: from the grounded side of the battery (not shown) at the main central office, over the tip conductor of the trunk line T, tip contacts of jack 10 and plug 15, upper normal contacts of keys FR, LK and BK, tip contacts of plug 50 and jack 51, through the called station loop, ring contacts of jack 51 and plug 50,

of the cord circuit.

lower normal contacts of key BK, winding of supervisory relay 84 and the serially connected resistances 31 and 30, in parallel, lowermost normal contacts of key LK, lower normal cgntacts of key FK, ring contacts of plug 15 and jack 10, and thence over the ring conductor of the trunk line T to battery (not shown) at the main central office. It will be noted that until the called subscriber responds the high and low impedance bridges are both connected across the cord circuit. As soon as the called station responds, however, the low impedance bridge is removed in a manner hereafter described. As is well known the circuit over the trunk line T, if said line leads to a manual otlice, may include a supervisory relay in a cord circuit (not shown) at the main central office. In case the main central ofiice is of the machine switching type, a relay (not shown) corresponding somewhat in function to the supervisory relay will be included in the machine switching circuit. While the supervisory relay 34, which is energized over the circuit just traced is controlled by the called subscriber, the supervisory relay at the main central office is controlled by the operator so that non-through supervision exists on incoming calls over the trunk line.

The supervisory relay 34, upon being energized, opens the circuit of lamp 40 and completes a circuit for relay 16 over a path extending from grounded battery, lower alternate contacts of relay 19, winding of relay 16, front contact and armature of relay 34, and sleeve contacts of plug 50 and jack 51 to ground. Relay 16, in attracting its outer right-hand armature, short circuits the non-inductive resistance 30, shunted about the supervisory relay 34; in attracting its outer left-hand armature opens the low impedance bridge through the windings of impedance coils 20 and 21; and in attracting its inner left-hand armature substitutes a connection to battery in place of the connection to battery through the sleeve circuit for the purpose of maintaining relay l8 energized. By the opening of the bridge of the low impedance coils 20 and 21, the shunt effect of the battery supply from the main central office of the subscribers instrument of the extension is obviated and a better transmission circuit is provided.

When the subscriber at the extension L hangs up his receiver, the circuit for relay 34, previously traced, is opened, and, consequently the supervisory relay 34 is deenergized. Relay 34, in retracting its armature, opens the circuit for relay 16, which relay thereupon deenergizes and closes at its outer left-hand contacts the low impedance bridge At the inner left-hand contacts of relay l6,"the original energizing circuit for relay 18 is again closed. The

the sleeve relay 'ifi l, upon deenergization, also completes a eircuitfl'froin ground, through the sleeve contacts of ack 51 and plug 50, armature and backcontact ol. relay 3e, and through the supervisory lamp 4&0 to grounded bat- 'tery,causing said lamp to glow to inform the, privz'iteybranch exchange operator that the connection may be taken down. Due to themaintenance of the high impedance bridge across the cord circuit, the machine switching apparatus at the main central oliice ispretented from releasing. It, on

the other handpthe main central oflice at remove plugs 15 and 50 from their respective jacks, thereby removing the bridge ,t'r'ointhe trunk circuit T and causing the release of'the switches at the main central officeor the deenergization of the supervisory 'rela'y thereat, depending on whether the main central otlice is of the machine switching or manual type, in the manner well known in theart.

Should a subscriber at an extension line, such asL desire to be connected to a subscriber at the main central otfice, the opera tion is asfollows: when thesubscriber removes his receiver from the switch hook, the line relay 52 is energized to close the cir cuit of the line lamp 53. The operator in response to the lighting of the line lamp may insert the plug 500i" the cord circuit G into" the jack 51, thereby disconnecting the line relay 52 and extinguishing the line lamp 53. As the relay 1.9 is not now energized, the battery supply may be traced from grounded battery,through the lower normal contacts of relay 19, impedance coil 21, through the winding of supervisory relay 3% and the serially connected resistances 30 and 31, in parallel,- lower normal contacts of key'BK, ring contacts of plug 50 and jack 5l,through the calling substation loop, tip contacts ofjack 51 and plug 50, upper normal contacts of key BK, impedance coil 20, and upper normal contacts ot relay 19 to ground. The supervisory relay 341 is energized over the circuit just traced and opens the circuit oi the supervisory lamp -10, which would otherwise beclosed over contacts oil plug 50 and jack 51.

The operator may nowactuate the listening key 11K, thereby connecting her set to the cord circuit, in the manner already de scribed, so that she is enabled to converse with the calling subscriber. Upon ascertaining that the subscriber desires a connection through the main central. ol'lice, the operator may restore her key Ll; to normal andsthen i sert the plug 15 into the jack 10.

A circuit is now completed from groun led battery, through resistance 13, sleeve contacts of jack 10 and plug 15, through the inner left-hand normal contacts of relay l6,

pperwinding of relay 1?, and windingof relay 18 to ground. Relay 17, being of the marginal type, does not operate at this time. ltclay 18 operates, however, and completes a circuit, previously traced, tor relay 19.

Relay 19, in operating, disconnects the bat- 11v .1 tery supply through the impedance COllS :20 and 21 so that the battery supply for the calling subscribers station will now be supplied di ectly from the main central oilice. Relay 19, in operating, I circuit for the energization o'l relay 16 over a path extending i'roin grounded battery, lower alternate contacts of relay 19, winding 01 relay 16, front contact and armature of relay 3-1, sleeve contacts of plug 50 and jack 51 to ground. Relay 16, in operating, now looks up over a path extending from grounded battery, lower alternate contacts oilifrelay 19, winding front contact and"inner right-handarmature of relay 16, upper inner armature and back contact of relay 17, and inner lower contacts of key LK to ground (provided key LK is in normal position).

Further action upon the part of the private branch exchange'operator will now depend on whether or not the main central oflice is manual otlice or an automatic'o'flice. In case the main central ofiice is a manual office, the line relay (not shown) associated with the trunk line T at the main central ollice will be energized as soon as the plug 15 is inserted into the jack 10 by n'ieans of the circuit, already traced, in cluding the station of the extension line L it the listening key LB: is in normal condition. It the key Lil is operated, however, as would normally be the case under the conditions being described, a circuit for said line relay would be completed as follows: from battery (not shown), througi said line relay (not shown), over the ring conductor ot the line T, ring contacts of jack 10 and also completes a v plug 15, lower normal contacts of key Flt.

lowermost alternate contacts of key LK, impedance coil 59, normal contacts of relay (51), upper innern'iost alternate contacts of key 11K, upper normal contacts of key FK,

tip contacts ct plug 15 and jack 10, and

thence over the tip conductor of the trunk line T to ground (not shown) '6 the main central office. The operator at the dstant central office being apprised through the erator, of the number desired, so that the operator at the main central office may complete the connection. lhe private branch exchange operator may then restore the listening key LK to normal so that the talking circuit will bethe same as previously traced for a call incoming from the main central oiiice.

In case the main central office is of the machine switching type, the operator at the private branch exchange, after insorted the plug 15 into the jack 19, and after having actuated the listening key may then actuate the impulse sender ti, number or times corresponding to the number desired at the main central office, for

the purpose of setting the machine switching apparatus'thercat. Upon the initial movement of the impulse sender or dial S, Oil normal contacts 69 thereoi are closed, thereby completing an obvious circuit for slow-to-release relay 69, which in turn completes an obvious circuit for relay 60. Relay 60, in attracting its right-hand armature, completesan obvious circuit for relay 58, which relay, in opera ing, opens the conductive connection, which had previously existed between the front and back conductors of the cord, through the operators set. Battery supply is now furnished to the extension L through the windings of relay 61 over a path which may be traced follows: from grounded battery, through the lower winding of relay 61, lower right-hand winding of repeating coil 99, lower alternate contacts of relay 58, uppermost contacts of key LK, and thence over the path previously traced through supervisory relay 84, and through the calling substation loop through contacts 99 'ot key LE, upper alternate contacts of relay 58, upper righthand winding of repeating coil 99, and upper winding of relay 61 to ground. Supervisory relay is held energized over this circuit during the actuation of the dial and relay 61 is now energized. Relay 61, at its armature and front contact, completes a holding circuit for relay 58 to maintain tl e connection thus established independently of the relay 69 whose onergization depends upon the dial being oil normal.

When relay 69 was energized, upon the initial movement of the dial, a dialing circuit was completed to the trunk line T through the tip conductor of said line over the tip contacts of the jack 10 and plug 15, upper normal contacts of key FK, upper alternatev contacts of key LK, left-hand alternate contacts of relay 60, interrupting contacts 70 of the dial S, lowermost alternate contacts of key LK, lower normal contacts of key FK, ring contacts of plug 15 and jack 1O tothe ring conductor of the trunk line T. The dial operates in the well-known manner to interrupt'the dialing circuit just traced and operates the machine switching apparatus at thev main central orhce. The dialing circuit, it will be noted, is independent of the supervisory circuit controlled over the extension L However, at the conclusion of the dialing operation, the operator for any reason should fail to restore to thelistening key LK, a holding circuit for the trunk line T will be provided through the impedance coil 59 inthe operators set, as this coil will be bridged across the tip and ring conductors of the trunk line 'il, lhis' bridge is closed by the dee ergization of the slow-to-release relay 68 which takes place a moment after thedialing operation ceases and'which results in the deenergization of relay 60. WVith the circuit in this condition the calling subscriber is able to converse with the called subscriber, even though the listening key Lli has not been restored, by reason of the fact that the left-hand winding of the induction coil 99 is bridged across the tip and ring conductors of the cord circuit in par allel with the impedance coil 59 as soon as the siow-to-release relay 68 is released. The repeating coil 99 serves to inductively couplc the 'front and back conductors through the operators set so that the subscribers can talk through the repeating coil. It will be noted that the circuit of the left-hand winding of the repeating coil 99 was held opened during the dialing operation by the relay so as to prevent the dialing impulses from being transmitted to the calling subscriber. The operator hearing the conversation the parties will thus be audibly in formed that she has failed to restore the ke LE to normal and will thereupon restore said key. a

the completion of conversation, the hanging up of the calling subscribers re ceiver causes the release of the supervisory relay 34cand if the trunk line leads to an automatic exchange, the machine switching apparatus by which the connection was set up at the main central office will be released immediately in the manner well known in the art. It on the other hand the main central otlice at which the line T terminatesis a manual oiiice,'the opening of the circuit at the station of the extension line L hereinhefore traced, results in the deenergization of supervisory relay in a cord circuit at the main central oliice, so that the operator at such oiiice will be informed that the connection is to be taken down. Relay 34, upon deenergization, opens at its armature front contact, the originalenergizing circuit for relay 16, Relay 16 remains energised, however, over the locking circuit previously traced and prevents the bridge through coils 29 and 21 from being placed across the cord circuit. Relay 34, in deenergizing, also completes a circuit for superrisory lamp 40 over the path previously traced. causing: said lamp to glow to inform the private rlilZlCll exchange operator that the connection is to be taken down.

In the event of a ringing over the trunk line T with the plug in the jack 10, relay 11 will operate during the ringing intervals, causing relay 1a to operate and lock up, and the lamp 12 to light in the manner previ ously described. The operator noting the lighting" oi? signal 12 may actuate key LK and inquire the Wants of the party calling and then remove plug 50 from jack 51. Actuation 01 key UK opens the locking circuit for relay 16, whereupon said relay 16 releases. as soon as relay 16 releases and closes its inner left-hand contacts, the cord circuit is in the same condition as when the operator answers an incoming call on the trunk circuit T, whereupon relays 17, 18 and 19 operate, relay it releases and signal 12 is erztinguished it if no recall signal, as has been described, has been transmitted, the private branch exchange operator, upon observing the glowing otthe lam all) in response to the hangiiuz; up of the receiver associated With the extension L will Withdraw the plugs 15 and 50 from their respective jacks. By withdrawing the plugs 15 and 50 the supen visory lamp 40 will be extinguished and the relays of the cord circuit will be restored to normal position.

In case the subscriber of the extension L should desire to be connected with another extension L of the same private branch e3:- cliange, the operator will insert the plug into the jack 5i and actuate the listening keyLl': as before llelay 19 of the cord circuit not being: o ie'ratcd, current is supplied through the impedance coils 20 and 21 for the callin suhscribefls transmitter and. to operate the supervisory relay 3 1. T 1e operator, upon ascertaining the number of the desired subscriber. will insert the plugg15 into the jack '15. The sleeve contact of jack 75 being grouni'led, the relays 17. 18 and 19 are not (me "tied over the sleeve circuit in v described, and, consesupplied the called the impedance ccilsQO 21. Relay l6 not uiergized by the supervis elay 3 du- 2. co ncction of this cha tor for the reason that the cir- -iit for relay. 16 do nds upon the energizathe preriou" quently, bat tor subscriber 'throiv ti on of relav 19v l. "fter swnalin" the cal ed subscrilfier in the Well-known manner bv the rrtuation of the hey E li and upon the response of the called subscriber conversation may be carried. on. The restoration of the receivers to the switch hooks of the calling and called subscribers, zauses supervisory la '1) ill to be energized to inform the operator that the :OIlYtlSzitlOD has terminated. The operator l'DEty TBIIIOVG the plugs from their associated jacks, whereupon all apparatus will be restored to normal pos ion.

Ret'erring to Fig. 2 1? the ins and assuming that the subscriber at the main central. office desires to be connected to a subscriber at one or the extensions, as, for en ample, the extension E of the private branch exchange, ringing current Will be transmitted over the trunk line T from the main central ollice. thereby cn 'zi j relay 111. Relay 1.11, upon encrgization, completes an obvious circuit -tor line lamp 11... and through the left-hand Winding oi ClllliP/TQEP tial relay 11'1". Relay 114E thereupon locks up throu 'a its left-hand Winding: thereby line other path extending through the upper contacts of jack 110, right-hand armature and contact and right-hand Winding of 114 to grounded battery. Relay 114, or the differential type, immediately releases due to the current flowing through its right-hand Winding. The inon'ientary lowered resistance ot the trunk sleeve circuit, by reason of the path through the righthand winding of differential relay 11%, allows suilicient current to flow through the cord circuit sleeve to e -ergize the marginal relay 117. circuit just traced, thereby completing a locking circuit for relay 11"? and an energizing circuit for relay 119 over a path en; uding from grounded battery, armature and front contact of relay 118, and therre in parallel, one path extending through the lower winding' and front contact and armature ot relay 117 to ground, and the other path extending through the Winding of refront relay being lay 3.19 to ground. Relay 117, in operating,

iin edance coil 120 upper alternate contacts of relay 119, back contact and outer left hand armature of relay 116, impedance coil 121 to the ring conductor of the c rd circuit C. A second bridge is thus established Belay 118 also energizesin the A1 then act fie the rim 'uency transmit ringing cm'z'ent it e at scrlb manneis' Well also open ates tact

12*" -e1 Jory "relay ii 1- of impedance bridge 11-:

1L0 and. to be rezneveu 75 CiTCUit. T142 high impec an 1 which is 07-? uch e vzuue as to the suaervls bled. fhe circuit e station of the extension fellows: 1 fiery (not shown) 2. I

ever the tip con .10 501' of the 113?" contacts of 1' 1 7 MOTH] throuqh a siml fine eubstatien loop, th 210k 151 and plug 150,

of key B133 the r r ces i3- and L nec ed mm 1 tip '2 are i Inv the inc. 1 ccntects e relay 128:, 3 :5 and jac I upper 11013211 1 con 1 conducts]? 0+ l'tew t Shawn) 1; supervisoryre 3 we ccntrel Ofthe 2e subs; Ibers stetien of J1; the supeivi relay 1033 can '7 1 be 1mncctiens peak 1 ccwermeszt cs 4 J. :1]. centame 11 H Pntclhi Jie iq omens fcuit Am? @135 its outer letcw n -eeance m'mess 1 5; of He: batiein'y cede d a b. railsmissl Y mended It W111 be oted, however, the

deter 66 14:0 no; ceased LO glow. The operater the high impedance coil 125 is bridged across the cord circuit as long as relay 117 is energized.

/Vhen thesubscriliier at the extension E hangs up his receiver, the circuit "tor relay 13 1, previously traced, is opened, and, consequently, the said relay 134: deenergized. Relay 1341, in retracting its armature, opens the circuit for relay 116, which relay thereupon deenergizes and closes at its outer left hand contacts the bridge, including coils 120 and 121, across the cord circuit. The relay 13 1, upon deenergization. also completes a.

circuit 'for lamp signal 140, causing said lamp to glow to inform the private branch exchange operator that the connection is to be taken down. Due to the maintenance of the bridge acros'sthe core circuit the machine switching apparatus at the main central'oflice ispreven't'edtrom releasing. If on the other hand the main central otlice at whichthe trunk line T terminates is a manual oflice, the supervisory relay in the cord circuit at the main central oflice will no prevented trom releasing.

The operator at the private branch exchange, noting the disconnect signal 140, may remove plugs 115 and 150 from their respe'cti've jacks, thereby removing the bridge from the trunk circuit T and causing the release of the switches at the main central ofiice or the glowing of the supervisory lamp thereat, depending on whether the main central office is of the machine switching or manualtype. I

Should-a subscriber on an extension line, such asiEfldesi're to be connected to a subscriber at the main central office, he will remove his receiver from the switch hook, whereupon the line relay 152 is energized to close the circuit of the linelamp 153. The operator in response to the signal will insert the plug 150 of the cord circuit C into jack 151, thereby disconnecting the line relay 152 and extinguishing the line lamp 153. i As relay 119 is not now energized, the battery supply may be traced from grounded battery, over the inner lower normal contacts of relay 119, impedance coil 121, lower normal contacts of relay 128 through the serially connected resistances 130 and 131 and the winding of relay 134 in parallel, lower normal contacts of key BK ring contacts of plug' 1 50 and ack 151, through the substatio n loop, tip contacts of jack 151 and plug 150,' upper normal contacts of key Blit uppermost normal contacts 01 relay 128, impedance coil 120 and upper inner normal contacts of relay 119 to ground] The supervisory relay 13 1 is energized over the circuit just traced and opens the circuit of the supervisory lamp 140 whichwould otherwise be closed over the sleeve contacts Off plug 150 and jack 151.

The operator may now actuate the listenthrough the main central ottice the operator may insert the plug 115 into the jack 110. A circuit is now con'ipleted through the winding of relays lll'and 118 over the path previously traced. Relay 118 energizes but relay 117 does not energize due to the high value of the resistance 113. Relay 118, in operating, completes an obvious circuit for relay 119. delay 119, in operating, disconnects the battery supply through the impedance coils 120 and 121, so that the current for the calling subscribers station will now be supplied directly tromthe main central ottice. Relay 119, as before described, short circuits the supervisory relay 126. Relay 119, in energizin also connects battery through its lower innermost contacts to the winding of relay 116, causing thereby the energization of relay 116. Relay 116 now locks up over a path extending from ground, lower inner normal contacts of key LK (provided key LK is in normal position), uppermost contacts of relay 117, inner right hand armature and front contact and winding or" relay 116, and front contact and lower inner armature of relay 119to grounded battery. 1

Further action on the part of the private branch operator will now depend on whether or not the main central oflice is a manual office or an automatic otlice. In case the main central oflice is ainanual ofiice the line relay (not shown) associated with the trunk T at the main central otlice will be energized as soon as the plug 115 inserted in the jack 110 by means of the tl'irough supervisory circuit, already traced, including the calling station of the extension E If the key L1? is operated, however, as would normally be the case under the conditions being, described, a circuit for said line relay would be completed in a manner similar to that previously. described inconnection with the description of 1 of the dra rings. Tile operator at the distant central otiice, b ing apprised by the line lamp (not shown) that a call has arrived, answers the call in the usual manner and the private branch exchange op erator, through her telephone. set which is connected to the cord circuit C over'the contacts of the key LK is able to converse with the operator at the main central otlice and inform the operator the number desired. The operator Oil the private branch exchange may then restore the listening key. LK to normal so that the talking circuit will be the same as previously tracedifor a call in coming from the main central otlice.

In case the main central oflice is of the machine switching type the operator, at the private branch exchange, after having inserted the plug 115 into the jack 110, and after having actuated the listening key LE may then actuate the impulse sender, S a numbe of times corresponding to the num ber desired at the main central ofice for the purpose of setting the machine switching apparatus thereat. Upon the initial movement of the impulse sender or dial S, the or normal contacts 169 thereof are closed, thereby completing an obvious circuit for slow-to-release relay 168, which in turn completes an obvious circuit for relay 160. Relay 160, in attracting its righthand armature, completes an obvious circuit for relay 158, which relay in operating opens the conductive connection which had previously existed between the front and back conductors of the cord circuit through the operators set. Battery supply is now furnished to the extension E through the windings of relay 161 over a path similar to that previously traced in connection with the description of Fig. 1 of the drawings. Supervisory relay 1% is held energized over this circuit during the actuation of the dial and relay 161 is now energized. Relay 161, at its armature and front contact, completes a holding circuit for relay 158 to maintain the connection thus established, independently of the relay 160 whose energization depends upon the dial being off normal. WVhen relay 160 was energized, upon the initial movement of the dial, a dialing circuit was completed to the trunk line T over a path similar to that previously traced in connection with Fig. 1 of the drawings. The dial operates in the well known manner to interrupt the dialing circuit just traced to set the machine switching apparatus at the main central office.

It will be noted that the dialing circuit is independent of the supervisory circuit controlled over the extension E However, at the conclusion of the dialing operation, if the operator fails to restore the listening key LK a holding circuit for the trunk line T will be provided through the im pedance coil 159 in the operators set as this coil will be bridged across the tip and ring conductors of the trunk line T This bridge is closed by the deenergization of the slowto-release relay 168, which takes place a moment after the dialing operation ceases and which causes the deenergization of the relay 160. With the circuit in this condition the calling subscriber is able to converse with the called subscriber even though the listening key has not been restored by reason of the fact that the left-hand winding of induction coil 199 in the operators set is bridged across the tip and ring conductors of the cord circuit in parallel with impedance coil 159 as soon as slow-to-release relay 168 has released. The repeating coil measles 199 serves to inductively couple the front and back conductors through the operato-rs so that the subscriber can talk through the repeating coil. It will be noted that the circuit of the left-hand winding of the repeating coil 199 was held open during the dialing operation by the relay 168 to prevent the dialing impulses from being transmitted to the calling subscriber. The operator is thus audibly informed that she has failed to restore key LE to normal, whereupon she restores said hey.

lit the completion of conversation, the hanging up of the calling subscribers receiver causes the release of the supervisory relay 13 1. If the trunk line T leads to an automatic exchange the machine switching apparatus by which the connection was set up at the main central office will be released in the manner well known in the art. If on the other hand the main central ofiice at which the line T terminates is a manual ofice the opening of the supervisory circuit above traced results in the deenergization of a supervisory relay (not shown) in the cord circuit (not shown) at the main central ofiice so that the operator at such ofice will be informed that the connection is to be taken down. Relay 134C, upon deenergization, opens at its armature and front contact the original energizing circuit for relay'116. Relay 116 remains energized, however, over the locking circuit previously traced and tins prevents the low impedance bridge including coils 120 and 121 from being placed across the cord circuit. Relay 134:, in deenergizing, also completes a circuit for supervisory lamp 140 causing said lamp to glow to inform the private branch exchange operator that the connection is to be taken down. Another circuit is completed at the same time for relay 128 over a path extending from grounded battery, lower inner alternate contacts of relay 119, winding of relay 128, front contact and outer right-hand armature of relay 116, upper inner normal contacts of relay 128,

ack contact and armature of relay 134E, sleeve contacts of plug 150 and jack 151 to ground. Relay 128 thereupon energizes, thereby disconnecting the front from the back cord and connecting ground. and battery to the tip and ring conductors respectively of the back cord. By this opening of the cord circuit, ringing current transmitted over the trunk T from the main central oflice will not in the event that the private branch exchange operator delays taking down the connection, be transmitted to the extension E with which the cord circuit is still associated to ring the bell at the subscribers station. The ringing response relay 111 bridged across the trunk circuit will respond to the ringing signal and will cause the lamp signal 112 to light during the intervals that ringing current is on thelinet Relay 114 will attempt to" operate in responseto the ringing signals but due to its differential winding being connected to ground in the cord circuit, it cannot lock up and will release as soon as ringing current is removed flbm the trunk line T I If the subscriber at the extension, i should recall by taking the receiver oil the hook before .the connection has been taken down, a circuit is provided for energizing the supervisory relay 134 due to the opera- [ion of relay 128, so that by flashing the switch hook the lamp 140 may be caused to glow to attract the attention of the operator. The supervisory relay 134:, when again energized by the subscriber removing his receiver before the private branch exchange operator takes down the connection, opens the circuit of the relay 128. Relay 128 releases and again closes the cord circuit so that the connection extends directly to the main central oflice and current for supervisory relay 134 is now applied therefrom. lVith the circuit in this condition, the subscriber may now extend one or more calls to the distant central office without the aid or intervention of the private branch ex change operator.

The operator noting the disconnect signal 140 may withdraw the plugs 115 and 150 from. their respective jacks, whereupon all apparatus associated with the cord circuit will be released and the bridge across the trunk line T 2 will be removed, causing thereby the release or restoration to normal of any apparatus which may be associated with the trunk circuit.

In case the subscriber on the extension E for example, should desire to be connected with another extension E of the same private branch exchange, the operator will in response to the lighting of the lamp 153 insert the plug 150 into the jack 151 and then actuate the listening key LK as before. Relay 119 of the cord circuit not being operated, current is suppliedthrough theimpedance coils 120 and 121 for the calling subscribers transmitter and for operating supervisory relay 134. The operator upon ascertaining the number of the desired subscriber will insert the plug 115 into the jack 175. The sleeve contact of the ack 175 be ing grounded, the relay 118 will not be en ergized over the sleeve circuit as in the case previously described, and consequently, the current for the called subscriber flows through the impedance coils 120 and 121. Relay 116 is not energized by the supervisory relay 134 during a connection of this character for the reason that the circuit of relay 116 depends upon the energization of cord circuit, the circuit for saidlamp extending from battery through said lamp 191, over the back contact and upper outer armature of relay 119, back contact and armature of relay 126, sleeve contacts of plug 115 and jack 151 to ground. When the called sub scriber of theextension E responds, the supervisory relay 126 which is included in the battery supply circuit to the called subscriber and which is not in this'instance short-circuited' over the lower contacts of relay 119 becomes energized and opens the circuit of the lamp 191, hen the two subscribers hang up. the supervisory relays 126 and 13% which are separately controlled by the two subscribers become deenergized and the lamps 191 and 140 are again caused to glow to give the disconnect signals. lVhile as above described the end of the cord circuit terminating in the plug 150 was used as the answering end and the end of the cord circuit terminating in the plug 115 was used as the calling end of the cord. circuit,

the cord circuit will function equally well for local connections if the plug 115 is used as the answering plug and the plug 150 as the calling plug, since the two ends of the cord circuits are symmetrical so far as local connections are concerned. For local con sections, it will also be noted, that the feature of opening the cord circuit by means of the relay 128 in response to the disconnect signal is not present. The operator noting the disconnect signals may remove the plugs 115 and 150 from their respective jacks thereby causing the restoration of all apparatus to normal.

lVhat is claimed is:

1. In a telephone exchange system, a trunk line. a SIIlJSCI'lbQIf S line. a cord circuit for interconnecting said lines, and means in the cord circuit and in the connected trunk circuit .to provide through supervision it calls are completed in one direction over said trunk line and to provide non-through supervision it calls are completed in another direction over said trunk line.

2. in a telephone exchange system, a trunk line. a subscribefis line, a cord circuit for intercomiecting said line arranged to provide through supervision it calls are completed completed in the other direction over said trunk line, and apparatus in the cord and connected trunk circuit cooperating. to convert automatically the cord circuit from one class of supervision to the other.

In a telephone exchange system. a trunk line, a subscribers line, a cord circuit for interconnecting said lines arranged to provide through or non-through supervision, a relay in said cord circuit, and means associated with said trunk circuit for energizing said relay only when a call is incoming over said trunk line to arrange said cord circuit for non-through superi'risionh 4. In a telephone exchange systenn'a two- Way trunk line, a subsorihers line, a cord circuit for interconnecting said lines arranged to provide through or non-through. supervision, a relay in said cord circuit arranged when energizedto adapt the cord circuit for nont-hrough supervision, and a relay associated with the trunk circuit for energizing the relay in the cord circuit only when a call is incoming over the trunk line.

5. In a telephone exchange system, a trunk line, a subscribefls line, a cord circuit for interconnecting said lines arranged to provide through or non-through supervision, a differential relay associated with the a trunk line, and a relay in said cord circuit cooperating with said differential relay for converting the cord circuit from one class of supervision to the other.

' 6. In a telephone exchange system, a trunk line, a subscribers line, a cord circuit for interconnecting said lines, a bridge for said cord circuit, a'relay associated with said cord clrcult adapted when actuated to connect said bridge to said cord circuit, and means associated with said trunk line for actuating said relay only when a call is incoming over said trunk line. I

' 7. In a telephone exchange system, a trunk line, asubscribers line, a cord circuit for interconnecting said lines, a bridge for said cord circuit, a relay, and a diiferential relay associated with said trunk line for actuating said relay to connect said .hridge to said cord circuit only when a call is incoming over said trunk line.

8. In a telephone exchange system, a trunk line, a subscribers line, a cord circuit for interconnecting said lines arranged to provide through or non-through supervision, signal current responsive means associated with said trunk circuit, a relay associated with said trunk circuit controlled thereby, and means in. said cordcircuit responsive to the actuation of said relay for converting said cord circuit from one class of supervision to the other.

In testimony whereof, I have signed my name to this specification this 18th dayof March, 1921.

RAND S. BAILEY. 

